A reverse-thread bolt ( sometimes called a left-hand or counter-thread bold) is precisely the same as a "regular" bolt with one key exception. On a reverse-threaded bolt, the ridges (or threads) wrap around the bolt cylinder in the opposite direction. In practical terms, this means that you must turn them in a counter-clockwise direction in order to tighten them, unlike standard bolts, which tighten in a clockwise fashion. They are less common than ordinary bolts and used in specialized situations.

Bolt Basics

All bolt threads have a helix, which is how they spiral up the bolt cylinder. When tightening a bolt, its helix will turn in one of two directions, clockwise and counter-clockwise; this is called handedness. Most bolts have a right-handed thread and turn in a clockwise direction as you screw them in.

If you look at the threads of such a bolt, they appear to angle upward to the right (this is called pitch). Reverse-thread bolts have a left-handed thread and turn in a counter-clockwise direction when tightened. The threads appear to angle up to the left on these bolts.

Why Use a Reverse-Thread Bolt?

Counter-thread bolts are used in special situations when a right-handed bolt would be either impractical or unsafe. Some of the most common uses include:

• Hardware: Applying pressure with your left hand or foot (think of a bike handle or pedal) would cause a normal bolt or screw to loosen. The same goes for machines, such as a circular saw blade that turns in a counter-clockwise direction.
• Automotive: The bolts that attach the tires to your car may also be reverse-threaded on the left side in order to prevent them from loosening due to the torque produced by the wheel motion. 
• Safety: Gas-supply valves on welding equipment turn in a counter-clockwise direction in order to differentiate them from the valve controlling oxygen supply.
• Plumbing and heating: Pipes often have both left- and right-handed coupling so that they can't work loose and cause leaks while in use.

Bolt Types

There are three common bolt types; each has its own special uses. They are differentiated by the shape of their head and the tip of their base.

• Hex bolts have a hexagonal head and a flat tip. They are used in construction and in some machinery. Typically, they are connected to a nut in order to create a tight hold.
• Lag bolts look similar to hex bolts, except that they have a pointed tip designed to bore into wood. These are used for construction projects, such as joining wooden support beams.
• Carriage bolts have a smooth, domed head and a flat tip. These are typically used in machinery where the connection must hold fast.

Bolts are usually made of steel, either stainless, galvanized, or zinc-plated. Steel is strong and resists corrosion. You can also find bolts made of chrome- or nickel-plated steel as well as brass and bronze. These highly polished metal fasteners are usually reserved for decorative purposes.